Embodiments of the present invention relates to a method of connecting an impeller to a shaft through a tie rod, a connection arrangement and a rotary machine.
In many technical fields, an impeller and a shaft are connected together so that they rotate integrally. In some applications, there is a need to transmit a torque (and power) from the impeller to the shaft; in some applications, there is a need to transmit a torque (and power) from the shaft to the impeller; in some applications, more in general, the torque (and power) is transmitted from the impeller or to the impeller depending on the operating condition of the machine.
A well known solution for connecting an impeller to a shaft in e.g. overhung configuration provides for a threaded blind hole at one end of the shaft, an axial through hole in the hub of the impeller, a fastener in the form of a bolt; the impeller is placed close to the shaft so that the hole of the impeller is aligned to the hole of the shaft, the fastener is inserted in the hole of the impeller, and it is tightened in the hole of the shaft to firmly connect the impeller to the shaft.
A disadvantage of this well known solution is that the impeller is weakened because of the axial through hole. In fact, any rotating impeller is stressed by the centrifugal forces that are proportional to the square of the rotation speed, and the axial through hole causes an increase in the intensity of this kind of stress with respect to the intensity of the stress in a solid impeller. Due to the increase in the stress, it is necessary to limit the rotation speed of the impeller and thus tip speed of its blades and thus, in case of e.g. a compressor impeller, its head.
This disadvantage applies fully to all solutions wherein the impeller has an axial through hole regardless of its size.
This disadvantage applies partially to all solution wherein the hub of the impeller has an axial blind hole regardless of its size.
Therefore, there is a general need to find improved solutions for connecting impellers to shafts.